1. Field of the Invention
The present invention relates generally to metal-clad or -enclosed switchgears enclosing control units for controlling and monitoring loads such as electric motors to protect the loads against overload or the like, and more particularly to a control unit withdrawing mechanism for the switchgears wherein each control unit is withdrawably disposed in each unit compartment of the switchgear and locked in the position.
2. Description of Prior Art
The metal-clad switchgear such as a control center generally has a box-shaped frame and the interior thereof defined by the frame is generally divided into a plurality of unit compartments by shelves disposed horizontally. Control units are withdrawably disposed in the unit compartments respectively. Each control unit includes a moulded case circuit breaker, an electromagnetic contactor, a control relay and the like. These control equipments are mounted on a common frame in order to perform controlling, protecting and monitoring the loads and other functions. A main cirucit and a control circuit of the control center are adapted to be automatically connected to and disconnected from a circuit disposed at the frame side when the control unit is placed in and withdrawn from the unit compartment.
Several types of control unit withdrawing mechanisms are well known in the art and have advantages and disadvantages. The control unit withdrawing mechanism is required to perform the following functions:
(1) The control unit can be placed in and withdrawn from the unit compartment along a guide rail.
(2) The control unit can securely be locked in its connection position and test position on the frame.
(3) The control unit can securely be locked on the frame when occupying the connection position so as to be prevented from slipping out of place due to the occurrence of an earthquake and the like.
(4) When moved from the connection position to the test position, the control unit can securely be stopped at the test position with ease.
A conventional control unit withdrawing mechanism will now be described with reference to FIGS. 14 and 15. Reference numeral 1 indicates a frame of the metal-clad switchgear. The interior of the frame 1 is divided into a plurality of unit compartments 3 by shelves 2. Control units 4 are withdrawably enclosed in the unit compartments respectively. One of the control units 4 is shown in the drawing. General L-shaped guide rails 5, are secured on opposite end portions of the shelf 2 respectively. A generally H-shaped positioning aperture 6 is formed in the center of the shelf 2. The positioning aperture 6 includes relatively wide front and rear engagement regions 6a and 6b and a relatively narrow guide region 6c communicating between the front and rear engagement regions 6a and 6b. Guide strips 7 formed on opposite ends of the bottom wall of the control unit 4 respectively are withdrawably attached to the guide rails 5 so as to be slidable therealong. A lever 8 is rotatably mounted on the control unit 4. A screw 9 is threadably engaged with the lever 8. Friction due to a spring 9a causes the lever 8 to be moved together with the screw 9. This rotational movement allows the lever 8 to be engaged with the front and rear engagement regions 6a and 6b of the positioning aperture 6. The lever 8 is provided with an engagement strip 8a which is caused to enter the guide region 6c by the rotational movement of the screw 9.
In the above-described control unit withdrawing mechanism, the control unit 4 is placed in the unit compartment 3 and then the screw 9 is threadably turned with a suitable tool in the direction of arrow A shown in FIG. 16 to thereby move the lever 8. The lever 8 is engaged with the rear engagement region 6b of the positioning aperture 6, thereby connecting a disconnector 10a of the control unit 4 to a power source bus 10b. Thus the control unit 4 is locked in the connection position. See FIGS. 16(a) and 16(b). When the control unit 4 is to be moved from the connection position to the test position (In this case, a disconnector 10a is disconnected from the power source bus 10b as shown in FIG. 17.), the screw 9 in the condition shown in FIG. 16(a) is turned by approximately 90.degree. in the direction opposite to arrow A to thereby disengage the lever 8 from the rear engagement region 6b of the positioning aperture 6. The screw 9 is further turned by approximately 180.degree. in the direction opposite to arrow A to thereby cause the engagement strip 8a to enter the guide region 6c of the positioning aperture 6. When the control unit 4 is withdrawn toward the test position, the engagement strip 8a is moved along the guide region 6c and collides with the end surface of the front engagement region 6a, thereby positioning the control unit 4 at the test position. See FIGS. 17(a) and 17(b). Subsequently, when the screw 9 is turned by approximately 270.degree. in the direction of arrow A, the lever 8 is engaged with the front engagement region 6a of the positioning aperture 6, thereby locking the control unit 4 in the test position. When the control unit 4 is to be withdrawn from the unit compartment 3, the lever 8 is turned so as to be positioned as shown in FIG. 18(a) and subsequently, the control unit 4 is withdrawn. See FIG. 18(b).
In relation to the above-described conventional construction to the aforementioned functions required for the control unit withdrawing mechanism, the functions (1)-(3) are performed by the conventional mechanism. The function (4), however, cannot be desirably performed in a reliable manner by the conventional control unit withdrawing mechanism. That is, the screw 9 is required to be turned in the range of wide angle (approximately 270.degree.) in the event that the control unit 4 is moved from the connection position to the test position. Consequently, troublesome operation of the lever 8 is required. Further, since the screw 9 is required to be turned in the range of wide angle, the turning of the screw 9 with the tool would be suspended although the engagement strip 8a of the lever 8 is not actually inserted in the guide region 6c. In this case, since the control unit 4 is not properly positioned, the control unit 4 can be moved over the test position and fall out of the unit compartment 3.
Furthermore, the prior control unit withdrawing mechanism is not provided with means for preventing the control unit from being moved between the connection and test positions in the condition that the moulded case circuit breaker is electrically closed. Accordingly, the control unit may be moved even where the moulded case a circuit breaker is electrically closed, resulting in a serious hazard.